Operation of lean burn engines, e.g., diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy, due to their operation at high air/fuel ratios under fuel lean conditions. Diesel engines, in particular, also offer significant advantages over gasoline engines in terms of their fuel economy, durability, and their ability to generate high torque at low speed.
Components of diesel engine exhaust subject to regulatory control are particulate matter (PM), nitrogen oxides (NOx), unburned hydrocarbons (HC), carbon monoxide (CO) and, recently, gases contributing to global warming. NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2). Depending on regulations, catalyst compositions and substrates on which the compositions are disposed are provided in diesel engine exhaust systems to convert these exhaust components to innocuous components.
The total particulate matter emissions of diesel exhaust are comprised of three main components. One component is the solid, dry, solid carbonaceous fraction or soot fraction. This dry carbonaceous matter contributes to the visible soot emissions commonly associated with diesel exhaust. A second component of particulate matter is the soluble organic fraction (“SOF”). The SOF can exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending upon the temperature of the diesel exhaust. It is generally present as condensed liquids at the standard particulate collection temperature of 52° C. in diluted exhaust, as prescribed by the standard measurement test, such as the U.S. Heavy Duty Transient Federal Test Procedure. These liquids arise from two sources: 1) lubricating oil swept from the cylinder walls of the engine each time the pistons go up and down; and (2) unburned or partially burned diesel fuel.
The third component of the particulate matter is the sulfate fraction. The sulfate fraction is formed from the oxidation of sulfur containing compounds in either the fuel or lubricating oil. The oxidized sulfur combines rapidly with water in the exhaust to form sulfuric acid. The sulfuric acid collects as a condensed phase with the particulates as an aerosol, or is adsorbed onto the other particulate components, and thereby adds to the mass of the total particulate matter.
As particulate mass emissions standards become more stringent and with introduction of particulate number emissions standards, it is believed that the filtration efficiency of diesel particulate filters will need to increase. However, an increase in filtration efficiency results in higher pressure drops. Alternatively engine builders may increase engine out NOx emissions and lower particulate emissions, a filter with less than 90% filtration efficiency may meet the standards. Particulate filtration applications in emerging markets such as China and Brazil may not require high-filtration efficiency.
The most common particulate filter used in mobile diesel applications for automobiles, trucks and buses is the wall flow ceramic filter, which comprises a honeycomb substrate with longitudinally extending walls defining channels that are alternately blocked on their inlet and outlet ends. The gas is forced through the honeycomb wall.
Cyclone particle collection involves removal of particulate based upon differences in the inertial mass of the particulate and gas when experiencing acceleration. A traditional cyclone is essentially a settling chamber in which gravitational acceleration is replaced by a centrifugal separating force where the acceleration might be as high 2500 g obtained by rotational movement. The ability of a cyclone to separate and collect particles depends upon the particular cyclone design, the properties of the gas and the gas particles, the amount of dust contained in the gas and the size distribution of the particles. (SAE 2005-01-3695), Particles that are effectively handled by cyclone typically have an average particle size of at least 5 microns. Cyclones are generally classified according to their gas inlet design, dust discharge design, their gas handling capacity, collection efficiency, and their arrangement. The most commonly used cyclone is the medium efficiency, high gas throughput (conventional) cyclone. Cyclones of this type are used primarily to collect coarse particles when collection efficiency and space requirements are not a major consideration. Collection efficiency for conventional cyclones on 10 μm particles is generally 50 to 80 percent.
Typically diesel particulates have particulates having an average particle size of about 0.5 microns and most diesel particulates are less than 1 micron in size. Moreover, diesel particulate is carbonaceous, and the carbonaceous particles have a low specific gravity. Accordingly, a typical cyclone arrangement for particle collection and removal would be ineffective for particulate removal required to remove sufficient quantities of particulate from lean burn engine exhaust.
There is a need, therefore, for a particulate filtration apparatus that is capable of removing small particulate matter from the exhaust gas stream of a lean burn engine exhaust, in particular, diesel engine exhaust. It would be desirable to provide a filter that can consistently deliver good filtration efficiency %, operate at relatively low pressure drop and not be susceptible to plugging over its lifetime, as compared to a wall flow filter or other partial filters currently in use.